Cancer is one of the leading causes of death in the developed world, with over one million people diagnosed with cancer and 500,000 deaths per year in the United States alone. Overall it is estimated that more than 1 in 3 people will develop some form of cancer during their lifetime. There are more than 200 different types of cancer, four of which-breast, lung, colorectal, and prostate—account for over half of all new cases (Jemal et al., 2003, Cancer J. Clin. 53:5-26).
Breast cancer is the most common cancer in women, with an estimated 12% of women at risk of developing the disease during their lifetime. Although mortality rates have decreased due to earlier detection and improved treatments, breast cancer remains a leading cause of death in middle-aged women, and metastatic breast cancer is still an incurable disease. On presentation, most patients with metastatic breast cancer have only one or two organ systems affected, but as the disease progresses, multiple sites usually become involved. The most common sites of metastatic involvement are locoregional recurrences in the skin and soft tissues of the chest wall, as well as in axilla and supraclavicular areas. The most common site for distant metastasis is the bone (30-40% of distant metastasis), followed by the lungs and liver. And although only approximately 1-5% of women with newly diagnosed breast cancer have distant metastasis at the time of diagnosis, approximately 50% of patients with local disease eventually relapse with metastasis within five years. At present the median survival from the manifestation of distant metastases is about three years.
Current methods of diagnosing and staging breast cancer include the tumor-node-metastasis (TNM) system that relies on tumor size, tumor presence in lymph nodes, and the presence of distant metastases (American Joint Committee on Cancer: AJCC Cancer Staging Manual. Philadelphia, Pa.: Lippincott-Raven Publishers, 5th ed., 1997, pp 171-180; Harris, J R: “Staging of breast carcinoma” in Harris, J. R., Hellman, S., Henderson, I. C., Kinne D. W. (eds.): Breast Diseases. Philadelphia, Lippincott, 1991). These parameters are used to provide a prognosis and select an appropriate therapy. The morphologic appearance of the tumor can also be assessed but because tumors with similar histopathologic appearance can exhibit significant clinical variability, this approach has serious limitations. Finally assays for cell surface markers can be used to divide certain tumors types into subclasses. For example, one factor considered in the prognosis and treatment of breast cancer is the presence of the estrogen receptor (ER) as ER-positive breast cancers typically respond more readily to hormonal therapies such as tamoxifen or aromatase inhibitors than ER-negative tumors. Yet these analyses, though useful, are only partially predictive of the clinical behavior of breast tumors, and there is much phenotypic diversity present in breast cancers that current diagnostic tools fail to detect and current therapies fail to treat.
Prostate cancer is the most common cancer in men in the developed world, representing an estimated 33% of all new cancer cases in the U.S., and is the second most frequent cause of death (Jemal et al., 2003, CA Cancer J. Clin. 53:5-26). Since the introduction of the prostate specific antigen (PSA) blood test, early detection of prostate cancer has dramatically improved survival rates; the five year survival rate for patients with local and regional stage prostate cancers at the time of diagnosis is nearing 100%. Yet more than 50% of patients will eventually develop locally advanced or metastatic disease (Muthuramalingam et al., 2004, Clin. Oncol. 16:505-16).
Currently radical prostatectomy and radiation therapy provide curative treatment for the majority of localized prostate tumors. However, therapeutic options are very limited for advanced cases. For metastatic disease, androgen ablation with luteinising hormone-releasing hormone (LHRH) agonist alone or in combination with anti-androgens is the standard treatment. Yet despite maximal androgen blockage, the disease nearly always progresses with the majority developing androgen-independent disease. At present there is no uniformly accepted treatment for hormone refractory prostate cancer, and chemotherapeutic regimes are commonly used (Muthuramalingam et al., 2004, Clin. Oncol. 16:505-16; Trojan et al., 2005, Anticancer Res. 25:551-61).
Colorectal cancer is the third most common cancer and the fourth most frequent cause of cancer deaths worldwide (Weitz et al., 2005, Lancet 365:153-65). Approximately 5-10% of all colorectal cancers are hereditary with one of the main forms being familial adenomatous polyposis (FAP), an autosomal dominant disease in which about 80% of affected individuals contain a germline mutation in the adenomatous polyposis coli (APC) gene. Colorectal carcinomas invade locally by circumferential growth and elsewhere by lymphatic, hematogenous, transperitoncal, and perineural spread. The most common site of extralymphatic involvement is the liver, with the lungs the most frequently affected extra-abdominal organ. Other sites of hematogenous spread include the bones, kidneys, adrenal glands, and brain.
The current staging system for colorectal cancer is based on the degree of tumor penetration through the bowel wall and the presence or absence of nodal involvement. This staging system is defined by three major Duke's classifications: Duke's A disease is confined to submucosa layers of colon or rectum; Duke's B disease has tumors that invade through the muscularis propria and may penetrate the wall of the colon or rectum; and Duke's C disease includes any degree of bowel wall invasion with regional lymph node metastasis. While surgical resection is highly effective for early stage colorectal cancers, providing cure rates of 95% in Duke's A patients, the rate is reduced to 75% in Duke's B patients and the presence of positive lymph node in Duke's C disease predicts a 60% likelihood of recurrence within five years. Treatment of Duke's C patients with a post surgical course of chemotherapy reduces the recurrence rate to 40%-50% and is now the standard of care for these patients.
Lung cancer is the most common cancer worldwide, the third most commonly diagnosed cancer in the United States, and by far the most frequent cause of cancer deaths (Spiro et al., 2002. Am. J. Respir. Crit. Care Med. 166:1166-96; Jemal et al., 2003, CA Cancer J. Clin. 53:5-26). Cigarette smoking is believed responsible for an estimated 87% of all lung cancers making it the most deadly preventable disease. Lung cancer is divided into two major types that account for over 90% of all lung cancers: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). SCLC accounts for 15-20% of cases and is characterized by its origin in large central airways and histological composition of sheets of small cells with little cytoplasm. SCLC is more aggressive than NSCLC, growing rapidly and metastasizing early. NSCLC accounts for 80-85% of all cases and is further divided into three major subtypes based on histology: adenocarcinoma, squamous cell carcinoma (epidermoid carcinoma), and large cell undifferentiated carcinoma.
Lung cancer typically presents late in its course, and thus has a median survival of only 6-12 months after diagnosis and an overall 5 year survival rate of only 5-10%. Although surgery offers the best chance of a cure, only a small fraction of lung cancer patients are eligible with the majority relying on chemotherapy and radiotherapy. Despite attempts to manipulate the timing and dose intensity of these therapies, survival rates have increased little over the last 15 years (Spiro et al., 2002, Am. J. Respir. Crit. Care Med. 166:1166-96).
These four cancers, as well as many others, present as solid tumors that are composed of heterogeneous cell populations. For example, breast cancers are a mixture of cancer cells and normal cells, including mesenchymal (stromal) cells, inflammatory cells, and endothelial cells. Several models of cancer provide different explanations for the presence of this heterogeneity. One model, the classic model of cancer, holds that phenotypically distinct cancer cell populations all have the capacity to proliferate and give rise to a new tumor. In the classical model, tumor cell heterogeneity results from environmental factors as well as ongoing mutations within cancer cells resulting in a diverse population of tumorigenic cells. This model rests on the idea that all populations of tumor cells have some degree of tumorigenic potential. (Pandis et al., 1998, Genes, Chromosomes & Cancer 12:122-129; Kuukasjrvi et al., 1997, Cancer Res. 57:1597-1604; Bonsing et al., 1993, Cancer 71:382-391; Bonsing et al., 2000, Genes Chromosomes & Cancer 82: 173-183; Beerman H et al., 1991, Cytometry 12:147-54; Aubele M & Werner M, 1999, Analyt. Cell. Path. 19:53; Shen L et al., 2000, Cancer Res. 60:3884).
An alternative model for the observed solid tumor cell heterogeneity derives from the impact of stem cells on tumor development. According to this model, cancer arises from dysregulation of the mechanisms that control normal tissue development and maintenance. (Beachy et al., 2004, Nature 432:324). During normal animal development, cells of most or all tissues are derived from normal precursors, called stem cells (Morrison et al., 1997. Cell 88:287-98: Morrison et al., 1997, Curr. Opin. Immunol. 9:216-21; Morrison et al., 1995, Annu. Rev. Cell. Dev. Biol. 11:35-71). Stem cells are cells that: (1) have extensive proliferative capacity; 2) are capable of asymmetric cell division to generate one or more kinds of progeny with reduced proliferative and/or developmental potential: and (3) are capable of symmetric cell divisions for self-renewal or self-maintenance. The best-studied example of adult cell renewal by the differentiation of stem cells is the hematopoietic system where developmentally immature precursors (hematopoietic stem and progenitor cells) respond to molecular signals to form the varied blood and lymphoid cell types. Other cells, including cells of the gut, breast ductal system, and skin are constantly replenished from a small population of stem cells in each tissue, and recent studies suggest that most other adult tissues also harbor stem cells, including the brain. Tumors derived from a “solid tumor stem cell” (or “cancer stem cell” from a solid tumor) subsequently undergo chaotic development through both symmetric and asymmetric rounds of cell divisions. In this stem cell model, solid tumors contain a distinct and limited (possibly even rare) subset of cells that share the properties of normal “stem cells”, in that they extensively proliferate and efficiently give rise both to additional solid tumor stem cells (self-renewal) and to the majority of tumor cells of a solid tumor that lack tumorigenic potential. Indeed, mutations within a long-lived stem cell population may initiate the formation of cancer stem cells that underlie the growth and maintenance of tumors and whose presence contributes to the failure of current therapeutic approaches.
The stem cell nature of cancer was first revealed in the blood cancer, acute myeloid leukemia (AML) (Lapidot et al., 1994, Nature 17:645-8). More recently, it has been demonstrated that malignant human breast tumors similarly harbor a small, distinct population of cancer stem cells enriched for the ability to form tumors in immunodeficient mice. An ESA+, CD44+, CD24−/low. Lin-cell population was found to be 50-fold enriched for tumorigenic cells compared to unfractionated tumor cells (Al-Hajj et al., 2003, Proc. Nat'l Acad Sci. 100:3983-8). The ability to prospectively isolate the tumorigenic cancer cells has permitted investigation of critical biological pathways that underlie tumorigenicity in these cells, and thus promises the development of better diagnostic assays and therapeutics for cancer patients. It is toward this purpose that this invention is directed.